No.... it don't just happen overnight.
The bacteria does enter the Koi through Gill flukes and flukes via the skin(under scales ect) liken it to mosquito's injecting malaria in humans. Also don't forget it is in the water all the time. If Koi should be scraped or injury on a sharp object then the immune layer of slime is breached it to can allow bacteria in.
The slime coat that you feel on the Koi is its defense to these types of attacks by bacteria ect. If conditions are poor(water quality) this may breech this defense. This is one reason salt is used it cause a heavier slime coat due to irritation to the skin. And if used in the right format then it can also have range of kill power to some parasites.........providing there not already non-sensitive to salt.
Now you don't always see the infection until it is to late (Ulcers/body fin streaking) also sitting on bottom all signs of stress from the bacteria. I have seen some Koi with Aeromonas and they do recover...but most cases it is the stress of this attack that cause secondary infections(systemic) internal renal failure occures....kidneys/liver and so forth. That is why we don't recommend using certain antibiotics due to it is hard on the renal organs that are ALREADY failing or stressed.
Once you see these types of signs they as a rule are in advance stages and is a tougher battle...........and may lose Koi to the stress factor as it weakens from the bacterial infections.